1. Field of the Invention
The present invention relates to a capsule toner, a method of manufacturing the capsule toner, and a two-component developer.
2. Description of the Related Art
An image forming apparatus for forming images by means of electrophotography comprises a photoreceptor, a charging section, an exposure section, a developing section, a transfer section, a fixing section, and a cleaning section.
The charging section electrically charges the surface of the photoreceptor. The exposure section applies signal light to the surface of the photoreceptor in a charged state to form an electrostatic latent image corresponding to image information. The developing section supplies toner contained in a developer to the electrostatic latent image formed on the surface of the photoreceptor to form a toner image. The transfer section transfers the toner image formed on the surface of the photoreceptor onto a recording medium. Moreover, the fixing section fixes the transferred toner image to the recording medium. Further, the cleaning section is constructed for example of a cleaning blade, and scrapes off residual toner remaining on the surface of the photoreceptor after toner image transfer by a blade to clean the surface of the photoreceptor.
Such an image forming apparatus effects image formation by developing an electrostatic latent image with use of, as a developer, a one-component developer containing toner or a two-component developer containing toner and carrier. The toner used therein takes the form of resin particles obtained by dispersing a colorant and a wax or the like acting as a release agent in a polyester-based binder resin which is a matrix, followed by performing granulation.
A kneading-pulverization method has been widely used to date as a toner manufacturing method. However, a pulverized toner consists of particles of irregular shape with many surface asperities, and the surfaces thereof in a pulverized state become toner particle surfaces without being treated, with consequent likelihood of lack of uniformity in surface composition. This makes it difficult to render the surface conditions of toner particles uniform. When toner particle surfaces are irregular-shaped and bears many asperities, then toner flowability is deteriorated, or fogging, toner scattering, or the like problem occurs due to lack of uniformity in toner composition.
In light of such a problem resulting from irregularity in shape of toner particle surfaces, instead of the kneading-pulverization method, there have been proposed various wet techniques whereby a toner is manufactured by mixing fluid dispersions of toner raw materials, followed by causing aggregation. However, in the case of the wet technique, a dispersion stabilizer and an aggregating agent are heavily used, wherefore part of such a component remains on the surfaces or in the interior of toner particles, with consequent deterioration in resistance to moisture and in charging characteristics. As a notable drawback, charging characteristics are likely to become unstable significantly.
Meanwhile, in keeping with the recent trend toward even higher image quality, toner has come to be designed to have an increasingly smaller particle size. This creates the tendency of an increase in the proportion of a toner with small particle size in fine powder form contained in a two-component developer. In a two-component developer containing a toner with small particle size, due to cracking or changes in shape in the toner with small particle size resulting from a stress caused within a developing device, there arise a toner-spent phenomenon on a carrier (contamination of a charge-applying member) and ensuing deterioration in charging properties of a developer. This adversely affects the processes of development and transfer, thus causing image quality degradation.
Hence, as a toner characterized by having excellent flowability, transfer properties, and so forth, being uniform in respect of charging performance, having high offset resistance, and in addition offering various advantageous capabilities, a capsule toner is proposed that is formed by applying a resin-layer coating to the surfaces of toner base particles.
However, in the capsule toner with a resin-layer coating, in general, fine resin particles whose heat resistance is higher than that of toner base particles is used to achieve blocking resistance improvement. In this case, the toner base particles cannot be melted readily, with consequent likelihood of occurrence of a low-temperature offset phenomenon. Furthermore, since the resin coating layer hinders a release agent from oozing from the interior of the toner base particles, a high-temperature offset phenomenon is likely to occur, wherefore a sufficiently wide range of temperatures enabling fixation (non-offset temperature range) cannot be attained.
In Japanese Unexamined Patent Publication JP-A 6-342224 (1994), there is disclosed toner particles composed of base particles and fine resin particles that adhered firmly thereto by means of mechanical impact force. In addition, in Japanese Unexamined Patent Publication JP-A 5-173357 (1993), there is disclosed a toner with a wax that adhered firmly to toner surface.
However, in the toner disclosed in JP-A 6-342224, a release agent is contained in the base particles. Therefore, the release agent is unable to exude readily to the surfaces of toner particles during a fixing process, with consequent difficulty of attaining adequate offset resistance. Furthermore, in the toner disclosed in JP-A 5-173357, the toner surface is coated with a wax, wherefore the blocking resistance is poor.